Method for electrodeposition of paint



y 13, 1969 G. G. STROSBERG METHOD FOR ELECTRODEPOSITION 0F PAINT Filed Dec. '7, 1966 INVENTOR.

3,444,065 METHOD FOR ELECTRODEPOSITION F PAINT Gordon G. Strosberg, Oak Park, Mich., assignor to Ford Motor Company, Dearborn, MiclL, a corporation of Delaware Filed Dec. 7, 1966, Ser. No. 599,953 Int. C]. 1301!; 5/02 11.8. Cl. 204-181 14 Claims This invention is concerned with an improvement in electrically induced deposition of paint from an aqueous dispersion thereof. In particular, it is concerned with the practice of this art with a common coating bath within and circulating between two or more coating units.

Electrically induced deposition of paint upon electrically conductive objects is disclosed by Allen E. Gilchrist in US. Patent 3,230,162 which is incorporated herein by reference. This method is one of anodic deposition and involves aqueous dispersion of water soluble amino compound and paint comprising a film-forming paint binder the predominant fraction of which is a synthetic polycarboxylic acid resin, immersing the object to be coated in the dispersion and utilizing such object as the anode of an electrodeposition cell, placing a cathode in contact with such dispersion spaced apart from the object to be coated, and providing a difference of electrical potential between such electrodes which causes a water resistant film of the resin to deposit upon the anode. The film deposit is essentially self-leveling and self-terminating due to the nature of the resin employed, such resin being a material that deposits in substantially direct proportion to the flow of current between anode and cathode and builds a film characterized in that its electrical resistance sharply increases with depth.

Anodic deposition is utilized herein to illustrate the instant method since it has gained wider industrial use and hence is more fully understood than cathode deposition. Resins suitable for cathodically depositable paints include in their molecular structure functional groups which when ionized provide ionic sites of positive charge and are attracted to the negative electrode of the electrodeposition cell. Examples of such groups are amine and substituted amine groups such as quarternary ammonium groups. Dispersion of such resins is effected by the addition of water-ionizable acidic dispersal assistants, e.g., water soluble carboxylic acid comprisng compounds and suitably buffered inorganic acids of which buffered phosphoric acid is exemplary. It will be recognized by those skilled in the art that the instant process is applicable to either anodic or cathodic deposition.

It is further known that two or more objects to be coated may be present in the coating bath at one time. It is also known that two workpieces may be simultaneously coated in the bath at different coating voltages, i.e., the difference of potential between Workpiece A and the cathode or cathodes of primary effectiveness in the coating of Workpiece A may vary from the difference of potential existing between Workpiece B and the cathode or cathodes of primary effectiveness in coating Workpiece B.

As a practical matter, coating workpieces of differnt size and shape within a single coating tank often provides assembly line problems both inside and outside the coating tank and this can be further complicated if it is desired to coat such different workpieces at different voltages.

In the instant method a common coating bath is circulated between two or more coating tanks. The coating tanks are interconnected in fluid communication relationship, preferably arranged in substantially parallel reice lationship, and provided with independent electrical means for inducing electrodeposition. Common or separate power supplies and common or separate replenishment feed and paint-mixing means may be employed. A common heat exchange unit can be used to serve common bath. Uniform finish can be obtained on both coating lines as a result of the common bath and pumping means utilized for bath transfer can be utilized to provide bath agitation thereby reducing or eliminating separate agitation means heretofore employed. Thus, the system has certain advantages over the operation of two completely separate units and certain advantages over the operation of two coating lines in a single unit or coating within a single unit with different coating conditions applied at different stations within the bath.

The process to which the instant invention is directed together with the method and apparatus comprisng the improvement herein claimed will be more fully understood from the following detailed description of an embodiment of this invention when read in conjunction with the accompanying schematic drawing.

Referring now to the drawing, a first electrocoating tank 10 is in fluid communication with a second electrocoating tank by means hereinafter described. Tank 10 is in electrical connection with ground and via conductor 11 with a negative terminal of DC. power source 13. Tank 10 contains electrocoating bath 15, e.g., a pigmented polycarboxylic acid resin and diisopro-panol amine in aqueous dispersion, said baths conventionally contain about 5 to about 12 wt. percent nonvolatiles. Immersed in bath 15 in tank 10 is an electrically conductive article 17 which is supported by a hanger 19 which in turn is supported by an overhead conveyor 21. Article 17 is electrically insulated from conveyor 21 by insulator 23. Article 17 is in electrical connection with a positive terminal of power source 13 via hanger 19, conductor brush 2 5, bus bar 27 and conductor 29. Another workpiece indicated generally by numeral 31 is shown approaching tank 10.

In this embodiment, electrocoating bath 15 passes from tank 10 into tank 110 via conduit 33, valve 35, conduit 37, heat exchange unit 39 wherein the bath is passed in heat exchange relationship with suitable heat exchange means, and conduit 41. A portion of the bath is continuously or intermittently removed from tank 110 via conduit 43 and introduced into paint dispersal unit 45 wherein replenishment feed is introduced to the system. Replenishment feed is returned to tank 110 from unit 45 via conduit 47.

Bath 15 is transferred from tank 110 to tank 10 via conduit 49, pump 51 and conduit 53. It is within the scope of this invention to provide a heat exchange unit in heat exchange relationship with the bath passing through conduit 53. The coating bath is advantageously circulated between tanks on a time cycle and at a rate sufiicient to prevent the establishment of any substantial chemical or physical gradient between the portion of the bath in tank 10 and the portion of the bath in tank 110 so that the bath composition and operating conditions associated therewith are the same in each tank permitting identical coating of each line of workpieces.

Tank 110 is in electrical connection with a negative terminal of power source 13 via conductor 55.

Immersed in bath 15 in tank 110 is an electrically conductive article 117 which is supported by a hanger 119 which in turn is supported by an overhead conveyor 121. Article 117 is insulated from conveyor 121 by insulator 123. Article 117 is in electrical connection with a positive terminal of power source 13 via hanger 119, conductor brush 125, bus bar 127 and conductor 129.

A difference of electrical potential between about 50 and about 500 volts is maintained between article 17 and tank and between article 117 and tank 110. The upper range of voltage is dictated by the ability of the resin deposited to resist electrically induced rupture. Commonly, this difference of potential will be between about 100 and about 250 volts in each instance. It will be understood that the difference of electrical potential between article 17 and tank 10 may be essentially the same as the difference of electrical potential between article 117 and tank 110. In another embodiment the difference of potential between the electrodes in contact with the bath in one tank is greater than the difference of potential between the electrodes in contact with the bath in the second tank, e.g., at least about volts greater. This may be desirable where the objects to be coated in one tank are relatively more difiicult to coat due to recessed and/ or shielded surfaces. In some instances, it may be found desirable to maintain a coating voltage in one tank that is 50 to 150 volts higher than in the other tank.

The term amino compound is employed herein to include both amines and ammonia. It is preferred to employ water soluble amines, e.g., ethanol amine, diisopropanol amine, morpholine, and others heretofore disclosed in the literature. Ammonia, if used, is preferably employed in conjunction with an amine with the latter comprising the major amount of the solubilizer requirements in that am monia is more volatile and can produce wider and more rapid changes in the pH of the bath. While the optimum temperature of the bath may vary somewhat with the coating composition employed, such baths are conventionally operated at a temperature in the range of about 65 F. to about 85 F.

In this application, painting by electrodeposition is meant to include the deposition of finely ground pigment and/or filler in the ionizable resin herein referred to as the binder, the deposition of binder without pigment and/ or filler or having very little of same, but which can be tinted if desired, and the deposition of other water reducible surface coating compositions containing the binder which might be considered to be broadly analogous to enamel, varnish, or lacquer bases, and the coating material for such deposition is termed a paint. Thus, the binder, which is converted to a water-resistant film by the electrodeposition and ultimately converted to a durable film resistant to conventional service conditions by final curing, can be all or vritually all that is to be deposited to form the film, or it can be a vehicle for pigmentary and/or mineral filler material or even other resins on which it exerts the desired action for depositing the film. Suitable resins include but are not limited to those specifically listed in US. Patent 3,230,162 to A. E. Gilchrist. The preferred resins for anodic deposition have an acid number between about and about 300 and an electrical equivalent weight between about 1,000 and about 20,000. The term electrical equivalent weight is employed herein to mean that amount of resin or resin mixture that will deposit per Faraday of electrical energy input. The condi tions, procedures, and calculations which can be employed to determine electrical equivalent weight are set forth in detail in the aforementioned US. Patent 3,230,162.

Having described one embodiment of the invention in detail, it will be understood that various changes and modifications can be made in the method without departing from the spirit and scope of the invention, as defined in the claims.

I claim:

1. In a process for coating a first electrode in an electrical circuit comprising said first electrode, a second electrode of opposite polarity with respect to said first electrode, and an aqueous coating bath between and in contact with said first electrode and said second electrode, which process comprises applying a difference of electrical potential between said first electrode and said second electrode with resultant initiation of direct current of electrical energy through said circuit while there is dispersed within said bath in ionized form an organic film-forming resin characterized in that it deposits as a film of essentially even depth upon said first electrode substantially directly proportional to said current and the result-ant film deposit exhibits an increase in electrical resistance to said current in direct proportion to the thickness of said film deposit, the improvement which comprises charging a first coat-ing tank with a portion of said coating bath, charging a second coating tank with a second portion of said bath, circulating said coating bath between said first tank and said second tank on a time cycle and at a rate sufficient to prevent the establishment of any substantial chemical or physical gradient between the portion of said bath in said first tank and the portion of said bath in said second tank, positioning said first electrode and said second electrode in spaced apart relationship and in contact with said bath within said first tank, providing a first difference of electrical potential between said first electrode and said second electrode sufficient to cause a direct current of electrical energy through said bath between said first electrode and said second electrode and effect electrodeposition of a film of said resin upon said second electrode, and positioning a third electrode and a fourth electrode in spaced apart relationship and in contact with said bath within said second tank, providing a second difference of electrical potential between said third electrode and said fourth electrode sufficient to cause a direct current of electrical energy through said bath between said third electrode and said fourth electrode and effect electrodeposition of a film of said resin upon said fourth electrode, the polarity relationship of said third electrode to said fourth electrode being the same as the polarity relationship of said first electrode to said second electrode.

2. The method of claim 1 wherein said first difference of potential is quantitatively the same as said second difference of potential.

3. The method of claim #1 wherein said first difference of electrical potential is quantitatively greater than said second difference of electrical potential.

4. The method of claim 1 wherein said first difference of electrical potential is at least 25 volts greater than said second difference of electrical potential.

5. The method of claim 1 wherein said film-forming resin is a synthetic polycarboxylic acid resin and is dispersed in said bath with a water soluble amino compound.

6. The method of claim 1 wherein said film-forming resin is a synthetic polycarboxylic acid resin, said resin is pigmented and is dispersed in said bath with a water soluble amine, and said first electrode and said third electrode when in contact with said bath are positively charged in relation to said second electrode and said fourth electrode.

7. The method of claim 1 wherein a stream of said bath from said first tank is continuously passed into said second tank and a stream of said coating bath from said second tank is continuously passed into said first coating tank.

8. The method of claim 1 wherein a stream of said bath from said first tank is intermittently passed into said second tank and a stream of said bath from said second tank is intermittently passed into said first tank in accordance with said time cycle and said rate.

9. In a process for coating an anode in an electrical circuit comprising said anode, a cathode and an aqueous coating bath between and in contact with said anode and said cathode, which process comprises applying a difference of electrical potential between said anode and said cathode with resultant initiation of a direct current of electrical energy through said circuit while there is dispersed Within said bath a water soluble amine and in anionic form an organic film-forming resin characterized in that it deposits as a film of essentially even depth upon said anode substantially directly proportional to said current and the resultant film deposit exhibits an increase in electrical resistance to said current in direct proportion to the thickness of said film deposit providing a corresponding decrease in said current in substantially direct proportion to increase in thickness of said fil-m deposit at said difference of potential, the improvement which comprises charging a first coating tank with a portion of said coating bath, charging a second coating tank with a second portion of said bath, circulating said coating bath between said first tank and said second tank on a time cycle and at a rate sufiicient to prevent the establishment of any substantial chemical or physical gradient between the portion of said bath in said first tank and the portion of said bat-h in said second tank, positioning said anode and said cathode in spaced apart relationship and in contact with said bath within said first tank, providing a first difference of electrical potential between said anode and said cathode sufficient to cause a direct current of electrical energy through said bath between said anode and said cathode and effect electro-deposition of a film of said resin upon said anode, and positioning a second anode and a second cathode in spaced apart relationship and in contact with said bath within said second tank, providing a second difierence of electrical potential between said second anode and said second cathode surficient to cause a direct current of electrical energy through said bath between said second anode and said second cathode and elfect electrodeposition of a film of said resin upon said second anode.

10. The method of claim 9 wherein said first difference of electrical potential and said second difference of electrical potential are each in the range of about to about 500 volts.

1 1. The method of claim 9 wherein said first difference of electrical potential and said second difference of electrical potential are each in the range of about to about 250' volts.

12. The method of claim 9 wherein circulation of said bath between said first tank and said second tank is continuous.

13. The method of claim 9 wherein said bath is passed in heat exchange relationship With a heat exchange medium in passing between said firs-t tank and said second tank.

14. The method of claim 9 wherein said first difierence of electrical potential is at least 50 volts greater than said second difference of potential.

References Cited UNITED STATES PATENTS 3,200,057 8/1965 Burnside et a1 20'4181 3,230,162 1/1966 Gilchrist 204181 3,355,373 11/1967 Brewer e-t al 20418'1 3,355,374 ll/1967 Brewer et a1 204l81 JOHN H. MACK, Primary Examiner.

H. M. FLOURNOY, Assistant Examiner. 

1. IN A PROCESS FOR COATING A FIRST ELECTRODE IN AN ELECTRICAL CIRCUIT COMPRISING SAID FIRST ELECTRODE, A SECOND ELECTRODE OF OPPOSITE POLARITY WITH RESPECT TO SAID FIRST ELECTRODE, AND AN AQUEOUS COATING BATH BETWEEN AND IN CONTACT WITH SAID FIRST ELECTRODE AND SAID SECOND ELECTRODE, WHICH PROCESS COMPRISES APPLYING A DIFFERENCE OF ELECTRICAL POTENTIAL BETWEEN SAID FIRST ELECTRODE AND SAID SECOND ELECTRODE WITH RESULTANT INITIATION OF DIRECT CURRENT OF ELECTRICAL ENERGY THROUGH SAID CIRCUIT WHILE THERE IS DISPERSED WITHIN SAID BATH IN IONIZED FORM AN ORGANIC FILM-FORMING RESIN CHARACTERIZED IN THAT IF DEPOSITS AS A FILM OF ESSENTIALLY EVEN DEPTH UPON SAID FIRST ELECTRODE SUBSTANTIALLY DIRECTLY PROPORTIONAL TO SAID CURRENT AND THE RESULTANT FILM DEPOSIT EXHIBITS AN INCREASE IN ELECTRICAL RESISTANCE TO SAID CURRENT IN DIRECT PROPORTION TO THE THICKNESS OF SAID FILM DEPOSIT, THE IMPROVEMENT WHICH COMPRISES CHARGING A FIRST COATING TANK WITH A PORTION OF SAID COATING BATH, CHARGING A SECOND COATING TANK WITH A SECOND PORTION OF SAID BATH, CIRCULATING SAID COATING BATH BETWEEN SAID FIRST TANK AND SAID SECOND TANK ON A TIME CYCLE AND A RATE SUFFICIENT TO PREVENT THE ESTABLISHMENT OF ANY SUBSTANTIAL CHEMICAL OR PHYSICAL GRADIENT BETWEEN THE PORTION OF SAID BATH IN SAID FIRST TANK AND THE PORTION OF SAID BATH IN SAID SECOND TANK, POSITIONING SAID FIRST ELECTRODE AND SAID SECOND ELECTRODE IN SPACED APART RELATIONSHIP AND IN CONTACT WITH SAID BATH WITHIN SAID FIRST TANK, PROVIDING A FIRST DIFFERENCE OF ELECTRICAL POTENTIAL BETWEEN SAID FIRST ELECTRODE AND SAID SECOND ELECTRODE SUFFICIENT TO CAUSE A DIRECT CURRENT OF ELECTRICAL ENERGY THROUGH SAID BATH BETWEEN SAID FIRST ELECTRODE AND SAID SECOND ELECTRODE AND EFFECT ELECTRODEPOSITION OF A FILM OF SAID RESIN UPON SAID SECOND ELECTRODE, AND POSITIONING A THIRD ELECTRODE AND A FOURTH ELECTRODE IN SPACED APART RELATIONSHIP AND IN CONTACT WITH SAID BATH OF ELECTRICAL POTENTIAL BETWEEN SAID THIRD ELECTRODE AND SAID FOURTH ELECTRODE SUFFCIENT TO CAUSE A DIRECT CURRENT OF ELECTRICAL ENERGY THROUGH SAID BATH BETWEEN SAID THIRD ELECTRODE AND SAID FOUTH ELECTRODE AND EFFECT ELECTRODEPOSITION OF A FILM OF SAID RESIN UPON SAID FOURTH ELECTRODE, THE POLARITY RELATIONSHIP OF SAID THIRD ELECTRODE TO SAID FOURTH ELECTRODE BEING THE SAME AS THE POLARITY RELATIONSHIP OF SAID FIRST ELECTRODE TO SAID SECOND ELECTRODE. 